The present invention relates to acicular ferromagnetic metal particles having a high coercive force and a method for the preparation thereof. More particularly, it relates to acicular ferromagnetic metal particles having a particle size of 0.1 to 1 .mu.m and a dimension of crystallite-size of not more than about 215 A in the effective thickness of the crystallite in the direction perpendicular to the reflecting plane (110) which have a high coercive force, preferably not less than 1,400 oersteds, and are useful for a high-density magnetic recording tape, video mother tape, or permanent magnet material, and a method for the preparation of the acicular ferromagnetic metal particles from specific .alpha.-ferric oxy-hydroxide particles.
The ferromagnetic metal particles comprises mainly iron element but may contain a small amount of other metal elements such as nickel, chromium, cobalt, copper, or the like in order to prevent oxidation of the particles. The ferromagnetic metal particles are hereinafter referred to merely as "ferromagnetic iron particles".
Generally, the ferromagnetic iron particles tend to have a higher coercive force with smaller particle size similar to the usual magnetic particles. From a practical viewpoint, however, the magnetic particles, particularly useful for magnetic recording medium, should have a particle size of 0.1 to 1 .mu.m because of easy handling (e.g. for prevention of combustion) and the better dispersibility into binders. Such practical magnetic particles have merely a lower coercive force than 1,000 oersteds, usually 500 to 800 oersteds.
For instance, according to a reduction method using an alkali metal borohydride, which is a representative method among the conventional methods for preparing ferromagnetic iron particles, there can merely be produced ferromagnetic iron particles having more than 1,000 oersteds only when the particle size thereof is not larger than 0.04 .mu.m (cf. U.S. Pat. No. 3,865,627). Thus, in the practical particle size as mentioned above, any ferromagnetic iron particle having a high coercive force has never been prepared by the conventional method.
It has recently been reported that ferromagnetic iron particles having a coercive force of 800 to 1,300 oersteds could be prepared by using a specific agent for prevention of sintering in a method for preparing iron particles comprising reducing with heating powdery materials to be reduced such as a metal iron oxide or oxalate (cf. U.S. Pat. No. 3,607,220). According to this method using an agent for prevention of sintering, however, the ferromagnetic iron particles thus obtained have unfavorable defects. For instance, when a magnetic paint is prepared from the ferromagnetic iron particles, the agent for prevention of sintering is reacted with resins used as a binder to cause gelation of the paint, and hence, such ferromagnetic iron particles are unfavorable as magnetic materials.
The present inventors have already found that ferromagnetic iron particles having the desired particle shape, e.g. having a good acicularity, can generally be prepared by reducing with heating an .alpha.-ferric oxyhydroxide particle [FeO(OH)] (hereinafter, referred to as "geothite particle") with a reducing agent, wherein a goethite particle prepared by a specific method is used.
The goethite particles are usually prepared by adding a basic agent, which is used for the purpose of precipitating ferrous hydroxide or insoluble ferrous salts, to an aqueous solution of ferrous salts and then introducing an oxygen-containing gas into the mixture. The present inventors have found that when the goethite particles are prepared by carrying out the above reaction in a specific alkali solution, the goethite particles can be reduced with heating without sintering thereof and thereby the original shape of the goethite particles can almost be maintained as is.
Aiming at the fact that the ferromagnetic iron particles prevented from sintering have a higher coercive force than that of the conventional ferromagnetic iron particles, further extensive studies have been effected. As a result, it has now been found that the desired ferromagnetic iron particles having an extremely high coercive force can be prepared from specific goethite particles which are prepared by using a fixed large amount of the basic agent.
An object of the present invention is to provide improved ferromagnetic iron particles having a high coercive force.
Another object of the invention is to provide ferromagnetic iron particles having a coercive force of more than 1,400 oersteds useful, particularly, as a high-density magnetic recording medium.
A further object of the present invention is to provide an improved method for the preparation of ferromagnetic iron particles having a high coercive force.
These and other objects of the invention will be apparent from the following disclosure.
According to the present invention, the desired ferromagnetic iron particles having a high coercive force can be prepared by reducing with heating goethite particles which are prepared by treating an aqueous solution of a ferrous salt with a large amount of a basic agent and followed by oxidation of the resulting reaction mixture.
That is, the goethite particles are prepared by adding with agitation an aqueous solution of a ferrous salt to an aqueous solution of a basic agent, so that the pH value of the mixture becomes not less than about 12, preferably not less than about 13.5, whereby ferrous hydroxide being precipitated, and then blowing an oxygen-containing gas (e.g. air) into the reaction mixture at room temperature or an elevated temperature (e.g. at 20-80.degree. C.), whereby ferrous hydroxide being oxidized to produce goethite, and isolating the precipitated goethite particles, washing with water and then drying.
The starting ferrous salt includes ferrous sulfate, ferrous chloride, ferrous bromide, ferrous acetate and is usually used in an amount of 0.2 to 0.5 mol/l based on the whole volume of the reaction mixture after adding the basic agent. The basic agent includes alkali metal hydroxides or carbonates (e.g. sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, potassium carbonate, lithium carbonate), alkaline earth metal hydroxides (e.g. calcium hydroxide, magnesium hydroxide, strontium hydroxide) and ammonium hydroxide, but alkali metal hydroxides are the most preferable from the viewpoint of the solubility and the pH value of the solution as explained hereinafter. The basic agent may be used in an amount of not less than 6 mol, preferably not less than 8 mol, more preferably not less than 10 mol, per 1 mol of the ferrous salt, The upper limit of the amount may be restricted by the solubility of the basic agent, and hence, the amount of the basic agent is usually in the range of 6 to 80 mol, preferably 8 to 30 mol, more preferably 10 to 20 mol, per 1 mol of the ferrous salt.
In the preparation of the goethite particles, a small amount of salts of other metals may be added to the aqueous solution of a ferrous salt. Suitable examples of other metal salts are sulfate, chloride, bromide or acetate of nickel, chromium, cobalt or copper, and these salts may usually be used in an amount of a few to several percents by weight based on the weight of the ferrous salts. The geothite particles containing other metal components can give the ferromagnetic iron particles having excellent anti-oxidation properties by treating it as described below.
The goethite particles thus obtained are dehydrated with heating at about 200 to 800.degree. C., whereby .alpha.-ferric oxide is produced, and the resulting ferric oxide is reduced with a reducing gas (e.g. hydrogen gas) at a temperature of about 340.degree. to 420.degree. C., by which the desired ferromagnetic iron particles having a high coercive force can be obtained.